Foundationalism is an accepted term in epistemology which refers to the justification of a belief by reference to an absolutely indubitable 'foundation' belief. The certainty attributed to the foundation is believed to provide equally certain justification for the further belief that is 'built' on that foundation, just as foundations provide secure underpinning for a building. Examples of such 'foundations' in Western philosophy include the appeal to God, Descartes appeal to the self-conscious experience of thought ('I think therefore I exist') as the basis of the rest of his reasoning, or the appeal to unanalysable atomised components of experience by some empiricists ('This is white') as the basis for further empirical beliefs.

Depending on their attitude towards the relationship between facts and values, some dualist thinkers appeal to factual foundations which are then taken to imply ethical positions, whereas others attribute a foundation to facts which values are assumed to lack. Both of these kinds of dualists in different ways depend on the fact-value distinction to assume that purely factual and purely ethical beliefs exist, rather than recognising the inextricable relationship between these two kinds of claims in human experience. If one falsely isolates ethical beliefs in this way, ethical beliefs must either be taken to be foundationally supported (ethical foundationalism) or to be merely coherent with other beliefs and lacking foundational support (ethical coherentism). In A Buddhist theory of moral objectivity, ethical foundationalism is identified as a key feature of the eternalist tradition in Western thought, and ethical coherentism as a key feature of the nihilist tradition.

It is worth reflecting on the limitations of the basic foundationalist metaphor. Beliefs are taken to be in some sense like the parts of a building that need to be 'supported'. Without such support the results are also assumed to be disastrous - the building will fall down, as warned in Jesus' parable about the foolish man who built his house upon the sands (Mt 7:24-27). The strength of this idea is that beliefs, like buildings, need to be adequate to conditions. However, beliefs are quite different from buildings in some other respects: they are neither so well-defined nor so inflexible. They are not so well-defined because they are composed of language, which is formed of shifting meanings which have an emotional as well as a cognitive component (see section on language), and they are not so inflexible because they function in the mental states of human beings who are constantly adapting to a changing environment. Beliefs are more like suits of clothes than buildings: suits of clothes, too, can be more or less adequate to our surroundings, but they also move around with us and express our physical and mental states much more directly.

Apart from the misleading nature of the metaphor, foundationalism is also mistaken in its epistemological assumptions. As finite creatures, we are incapable of understanding an absolute justification, nor is there anywhere in finite experience that an absolute justification could come from. For example, religious experiences may or may not come from God in some sense, but given that they happen to people, even if they do they cannot convey any perfect knowledge. Even if we assume with the foundationalist that a foundation has been discovered, the moment we attempt to 'build' on that foundation to deduce further beliefs, the certainty of the foundation is lost because it must be compromised by its mixture with other premises when it is applied to experience. For example, even if we accept that Descartes established with certainty that he existed in a moment of self-conscious thought, the linkage of that thought with the next thought as the same self remains in doubt, together with any other knowledge held by that self which depends on any consistency through different experiences.

Foundationalists in effect depend on faith, of a type which associates a finite experience with an absolute and dogmatic justification. Foundationalism is an attempt to adopt the viewpoint of God whilst remaining human: an attempt which is obviously delusory. Yet many of those who can easily see its delusoriness when it appears in its grosser forms continue to appeal to it implicitly in their epistemological assumptions: whether this involves adopting a foundational view of 'self-evident facts', appealing to the enlightenment of the Buddha as the basis of human morality, or adopting an implicitly Platonic appeal to essential truths known through reason.